Fuel storage containment systems at gasoline dispensing facilities (GDF's) (i.e. gasoline stations) suffer from over-pressurization caused by fuel vaporization and thermal expansion, especially with high volatility wintertime fuels. Over-pressurization can be the cause of polluting gaseous emissions of fuel components to the atmosphere, soil, and groundwater because the various parts of fuel storage containment systems at GDF's are rarely, if at all, perfectly tight. Most often leakage can occur through fueling nozzle valves, fittings, pipe junctions, relief valves, and seals. The problem can be exacerbated by the recent and ongoing proliferation of vehicles equipped with on-board refueling vapor recovery (ORVR) systems which can cause some types of existing fuel dispensers with vapor recovery capability to ingest excess air during vehicle refueling, thereby promoting more evaporation and pressurization in the containment system.
The problem is substantially reduced for fuel dispensers equipped with passive “balance” type vapor recovery systems. In this case, air and vapor ingestion is significantly restricted by the combination of a nozzle to vehicle fill pipe seal which exists during dispensing and the ORVR equipped vehicle vapor seal which exists within the ORVR system, thereby preventing return vapor or air flow back into the fueling nozzle and, therefore, the fuel storage containment system. Under these conditions liquid fuel is dispensed (removed) from the containment system and little or no fuel vapor or air is returned to the containment system so the vapor space increases without a corresponding increase in vapor and air mass. Therefore the pressure in the system tends to be reduced. This substantially alleviates the over-pressurization problem in the containment system. But when no or only a few ORVR vehicles are refueled over many hours, for instance, as typically can occur during nighttime at a GDF, the containment system can still become over pressurized as described above.
The problem is more severe for dispensers equipped with active “vacuum assist” type vapor recovery systems. In this case, when ORVR vehicles are refueled, there is no seal between the nozzle and the vehicle fill pipe. A dispenser vacuum pump creates a vacuum at the nozzle to draw in fuel vapors which, for non-ORVR vehicle refuelings, are normally expelled from the vehicle's tank. But for ORVR vehicle refuelings, vapors are not expelled from the vehicle. Therefore, ambient air is ingested into the fuel storage containment system in place of fuel-rich vapors. This air is returned by the vacuum pump and vapor piping to the containment system tank(s). The returned air promotes excessive liquid fuel vaporization within the tank(s), resulting in over-pressurization of the system. One improvement which can reduce this problem is disclosed in U.S. Pat. No. 5,782,275, Jul. 21, 1998, “Onboard Vapor Recovery Detection”, Gilbarco, Inc. Another is disclosed in U.S. patent application Ser. No. 2002/0000258 A1, Jan. 3, 2002, “Dispenser with Radio Frequency On-Board Vapor Recovery Identification”, Dresser Inc. This apparatus senses the absence of fuel vapors during refueling and shuts off the vacuum pump while refueling ORVR equipped vehicles. This significantly reduces the amount of air and residual vapors returned to the containment system during refueling. Therefore, an ORVR detection equipped vacuum assist dispenser affects the containment system in a similar manner as a balance type dispensing system, significantly reducing the over-pressurization problem.
Various other means have been disclosed in patents and are used in practice to effect similar outcomes in order to handle ORVR equipped vehicles without causing excessive over-pressurization of the fuel storage containment system. All of these types of apparatus and methods are considered to be various types of “ORVR compatible” systems.
But all of these systems suffer from a common problem. When there is little or no refueling activity, evaporation and thermal expansion can still occur, causing over-pressurization and subsequent slow leakage of polluting containments into the environment. The California Air Resources Board (CARB) has promulgated regulations addressing this general problem. The regulations appear under the general title of Enhanced Vapor Recovery (EVR) system requirements. In part, they require that the containment system pressures remain below certain levels relative to ambient atmospheric pressure to limit the amount of slow leakage of pollutants into the environment.
An existing solution to the problem is to add a “vapor processor” onto the containment system to remove excess air from the containment system (“membrane separators”) or excess fuel vapors and air (“combustion systems”). But these methods are generally intended as high capacity, primary systems with capability beyond the needs of this residual over-pressurization problem and are expensive, complicated, and of limited reliability. They also emit low levels of pollution themselves during normal operation and have the potential to emit high levels of pollution under failure mode conditions. An example of such a device is shown in U.S. Pat. No. 5,985,002, Nov. 16, 1999, “Fuel Storage System with Vent Filter Assembly”.
The disclosed invention solves this residual over-pressurization problem for ORVR compatible, vapor recovery dispensing systems by controlling and limiting excess containment system pressures during periods of low fueling activity. It does this in a simple, low cost, reliable manner and in normal operation, no pollutants are emitted by the apparatus. It is applicable to all the types of vapor recovery equipped dispensing systems described above.